hook into 'build' task only if using Bundler tasks

https://github.com/sunaku/md2man/pull/7#issuecomment-9467621

Thanks to Postmodern for raising this issue.
ensure 'binman:web' task works the first time thru
workaround for md2man 1.6.0: must require it first
Version 3.2.1 (2013-05-04)